It is well known that automobile wheels are usually fixed on the axles by stud nuts and for the sake of safety, the stud nuts are usually screwed extremely tight. This makes loosening a wheel from an axles a very tough job for drivers having no power tool at hand. Nowadays, the most powerful bolt/nut loosening device generally available in the market is the pneumatic tool which, in one hand, is bulky in size and thus heavy in weight to be carried on a car, and on the other hand, is expensive for it requires an air compressor to supply pressurized air.
To overcome such a problem, it has been developed several kinds of electrically-driven devices suitable for being carried in an automobile, especially a passenger car, for loosening/tightening wheel stud bolts/nuts. Examples are U.S. Pat. Nos. 4,727,780, 4,920,831 and 5,305,161. The device of '780 patent requires manual intervention to apply a torque impulse to a nut or bolt to be loosened/tightened. This is apparently quite inconvenient.
The devices disclosed in '831 and '161 patents use a centrifugal clutch to control the application of torque impulse to the bolt or nut. The torque impulse is transmitted through a torque source to an output shaft through a coupling member which is angular position adjustable by control elements which are separated from and controlled by the centrifugal clutch. In a first position of the centrifugal clutch, the control elements allow the coupling member to engage the output shaft and in a second position of the centrifugal clutch, the engagement is disabled.
The use of separate control elements makes the assembly and operation of the wrench of this type difficult and complicated. A similar device with two control pins integrated on the centrifugal clutch to overcome such a difficult is disclosed in U.S. Pat. No. 4,947,939, entitled "Structure of Motorized Screw Bolt Driving Tool". The tool disclosed in '939 patent is, quite obviously, an improvement over the wrenches disclosed in '831 and '161 patents. The tool of '939 patent, although effective in overcoming the problem mentioned above, has several disadvantages. For example, the '939 patent discloses a structure which has a spindle holder separated from a swivel member. The spindle holder is connected to the spindle of a motor and the swivel member is keyed to the spindle holder so that the rotation of the motor spindle is transmittable to the swivel member. Since devices of this kind are intended to loosen over-tightened bolt or nut, a Great torque impulse is always transmitted through the structure thereof and under this situation, the key which is used to connect the spindle holder to the swivel member is very easy to break for such a great torque has to be solely born by the key which has only a small cross sectional area to support the great torque.
Furthermore, all the patents mentioned above has the disadvantage of unsuitability of operating in a dark environment, such as at night without lightening, so that separate lightening is required for using such devices, for example at night.
It is therefore desirable to provide an improved device for loosening/tightening bolts/nuts which overcomes the problems mentioned previously.